Is the end in sight for Sony's laser blues?

Sony's announcement last week of delays to the PlayStation 3 (PS3) in Europe was blamed solely on the difficulties in making one component: the blue laser diode for the built-in Blu-ray player.

Mass production of blue laser diodes is tricky. Light-emitting diode (LED) maker Nichia, which currently makes 80% of the global blue laser diode supply, reported that its yield rate for blue laser diodes has reached 30% - still behind demand.

Blue light's shorter wavelength means it can be focused on smaller objects. Optical discs such as CDs and DVDs have a spiral track of "pits"; each pit represents a digital bit. The more pits per disc, the more data. A CD (read with red laser light) has pits 0.5 microns wide; in Blu-ray and HD-DVD discs, the pits are 0.25 microns wide, and the tracks only a quarter as wide as on a CD. Only blue lasers can resolve those. Hence a double-layer hi-def DVD can store up to 50GB, but a single-layer CD only manages 0.74GB. But first, make your blue light diode (either a laser, which includes a mirror to amplify one particular frequency of generated light, or an LED, which generates a range of similar frequencies all at once). That's the problem.

But one man knows how to solve it. "The problem of yields [in producing blue laser diodes] is to do with the substrate used to grow the semiconductor material," says Professor Shuji Nakamura. And he should know: he's the inventor of blue, green and white LEDs, and the blue laser diode. He made these breakthroughs in the 1990s while an engineer at Nichia and was last week awarded the 2006 Millennium Technology Prize.

Blue LEDs and laser diodes consist of two thin layers of the semiconductor gallium nitride (GaN), with a layer of indium sandwiched in between. By adding different impurities, the two layers of GaN crystals are given slightly different properties: one layer (the n-type) conducts electrons, and the other (the p-type) contains positively charged "holes" as a result of electrons being soaked up by the added impurities.

The electrons in the n-type material can cross the junction and fall into the holes in the p-type equivalent. When this happens, the electron loses some energy. The lost energy is given off as a photon of light; its colour depends on how much energy is lost. As with all thin films of semiconductors, you need a good substrate to grow those crystal wafers on - one in which the spacing between atoms in the film matches that in the substrate material underneath. "At the moment, 'free-standing' gallium nitride layers are used as the substrate for growing GaN semiconductors," explained Nakamura. "And that's why the yield is not good."

A 'free-standing' GaN layer refers to a thin sheet of non-crystal GaN. It is deposited onto another substrate (for example, sapphire), and is then peeled off to act as the working surface for growing GaN semiconductor devices. This is cheaper and easier than using bulk crystals, but suffers from structural imperfections that can affect yield.

"The solution is to use bulk gallium nitride crystals instead," says Nakamura. Bulk silicon crystals form the substrate for conventional semiconductor devices. Single crystals are the ideal starting material, with perfect structural features. "This could improve the yield by 10-20 times," Nakamura says.

However, growing single GaN crystals is difficult; producing them in bulk is trickier still. They are prone to defects, which was why, until Nakamura's breakthrough, GaN had for decades been an outsider in the race for blue light. Despite Nichia and Sony having cross-licensed their patents to speed development of blue diodes in April 2004, neither has cracked the problem of pushing yields up.

Meanwhile, one of Nakamura's research interests is to develop techniques to produce bulk GaN crystals. While the commercial suppliers of blue laser diodes try to improve the yields, Nakamura might once again rise to win the race.

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